def get_discount(level):
    if level == 1:
        "大量計(jì)算代碼"
        discount = 0.1
    elif level == 2:
        "大量計(jì)算代碼"
        discount = 0.2
    elif level == 3:
        discount = 0.3
    elif level == 4:
        discount = 0.4
    elif level == 5:
        discount = 0.5
    elif level == 6:
        discount = 3 + 2 - 5 * 0.1
    else:
         return '等級(jí)錯(cuò)誤'
    return discount

大家都知道，這樣大量的if ... elif...代碼非常難看，也很難維護(hù)。并且每個(gè) if 的內(nèi)部有很多代碼。這個(gè)函數(shù)就會(huì)被拉得非常長。

有一些同學(xué)知道，可以使用字典來改寫這個(gè)太長的 if 判斷：

def parse_level_1():
    "大量計(jì)算代碼"
    discount = 0.1
    return discount

def parse_level_2():
    "大量計(jì)算代碼"
    discount = 0.2
    return discount

def parse_level_3():
    "大量計(jì)算代碼"
    discount = 0.3
    return discount

def parse_level_4():
    "大量計(jì)算代碼"
    discount = 0.4
    return discount

def parse_level_5():
    "大量計(jì)算代碼"
    discount = 0.5
    return discount

def parse_level_6():
    "大量計(jì)算代碼"
    discount = 3 + 2 - 5 * 0.1
    return discount

discount_map = {
 1: parse_level_1,
  2: parse_level_2,
  3: parse_level_3,
  4: parse_level_4,
  5: parse_level_5,
  6: parse_level_6,
}

discount = discount_map.get(level, '等級(jí)錯(cuò)誤')

但今天我學(xué)到的這個(gè)方法，比用字典更簡單。我們先來看它的效果：

@value_dispatch
def get_discount(level):
    return '等級(jí)錯(cuò)誤'

@get_discount.register(1)
def parse_level_1(level):
    "大量計(jì)算代碼"
    discount = 0.1
    return discount

@get_discount.register(2)
def parse_level_2(level):
    "大量計(jì)算代碼"
    discount = 0.2
    return discount

@get_discount.register(3)
def parse_level_3(level):
    "大量計(jì)算代碼"
    discount = 0.3
    return discount

@get_discount.register(4)
def parse_level_4(level):
    "大量計(jì)算代碼"
    discount = 0.4
    return discount

@get_discount.register(5)
def parse_level_5(level):
    "大量計(jì)算代碼"
    discount = 0.5
    return discount

@get_discount.register(6)
def parse_level_1(level):
    "大量計(jì)算代碼"
    discount = 3 + 2 - 5 * 0.1
    return discount


discount = get_discount(3)
print(f'等級(jí)3的用戶，獲得的折扣是：{discount}')

運(yùn)行效果如下圖所示：

這樣寫，比用字典的方式更直觀，比直接用if ... elif...更簡潔。

那么，這個(gè)裝飾器value_dispatch是怎么實(shí)現(xiàn)的呢？密碼就藏在這個(gè)開源項(xiàng)目EdgeDB的源代碼[2]中，核心代碼只有 20 多行：

并且，還能夠?qū)崿F(xiàn)或查詢。例如用戶等級(jí)為 2 或者 3 的時(shí)候，折扣都是 0.2，那么代碼可以寫成：

@get_discount.register(2)
@get_discount.register(3)
def parse_level_2(level):
    "大量計(jì)算代碼"
    discount = 0.2
    return discount

運(yùn)行效果如下圖所示：

它這個(gè)代碼目前只能實(shí)現(xiàn)相等的查詢。但其實(shí)只要對(duì)這個(gè)代碼稍作修改，我們就能實(shí)現(xiàn)大于、小于、大于等于、小于等于、不等于、in等等判斷。如果大家有興趣的話，請(qǐng)?jiān)谖恼孪旅媪粞裕覀兠魈炀蛠碚f說怎么對(duì)這個(gè)代碼進(jìn)行改造，實(shí)現(xiàn)更多的邏輯判斷。

參考文獻(xiàn)

[1] EdgeDB: https://github.com/edgedb/edgedb

[2] 源代碼: https://github.com/edgedb/edgedb/blob/master/edb/common/value_dispatch.py